Thyratron switches are used to switch energy stored in capacitors or pulse forming networks in order to drive high voltage pulse power devices such as pulse generators, lasers, radar transmitters, and particle accelerators. The so-called pseudospark and back-lighted thyratron (BLT) switch, a novel type of switch recently developed in both Europe and America, has, for many typical switching applications, required elaborate triggering means to achieve the desired switching characteristics. Some of these means involve unacceptable complications in switch support circuitry for what is intended to be an easily-applied, commercial device; others employ triggering devices which are incompatible with inclusion in a hermetically-sealed device.
Triggering refers to the process of closing the thyratron switch by initiating the electrical discharge in the switch so that the switch conducts current. In the present instance, triggering refers to closing a type of electronic switch comprised of two (or sometimes more) electrodes in an evacuated envelope filled with a low pressure gas for supplying electrons and ions. Specifically, for the purpose of describing this invention, it relates to a switch called a "pseudospark" switch, which refers to an electrically-triggered version of the switch. A "back-lighted thyratron," switch refers to an optically-triggered version of this switch. The invention concerns an electronic trigger that simplifies and improves the triggering method for the pseudospark device.
Switches of pseudospark or back-lighted type can be triggered in several different ways, and it is true that, once triggered, these switches conduct with short delay and little jitter. The problem, however, is (in common with other "cold-cathode" devices) that long and erratic breakdown delay times and excessive "high impulse ratio" trigger breakdown voltages may be encountered when establishing the trigger discharge itself in the first place.
In the absence of a heated thermionic cathode or other copious source of electrons, there may be relatively few electrons emitted or favorably placed prior to trigger breakdown to cause a triggering discharge of the desired characteristics to take place. Therefore, to meet the often simultaneous and conflicting requirements for short delay, low jitter, long life, and reasonable triggering requirements, some enhancement of the cold cathode switch triggering arrangements is generally necessary, particularly where the new pseudospark approach is intended to retrofit an application originally served by a hot cathode tube.
Many triggering methods and circuit variations have been devised to improve pseudospark/BLT triggering performance. A review is presented in "High Power Hollow Electrode Thyratron-Type Switches," K. Frank, E. Boggasch, J. Christiansen, A. Goertler, W. Hartmann, C. Kozlik, G. Kirkman, G. Braun, V. Dominic, M. A. Gundersen, H. Riege and G. Mechtersheimer, IEEE Tans. Plasma Science, published in 1988. The methods described for triggering include a pulsed-glow discharge or charge injection trigger, and a slide-spark (surface-discharge) trigger. These triggering methods, which are those used in the pseudospark switch, are also described in the article "High Power Hollow Electrode Thyratron-Type Switches," by the same authors in the Proceedings, Sixth IEEE Pulse Power Conference, June 1987. These articles present reviews of the state of the art.
Additional descriptions of trigger methods are contained in the articles J. Christiansen and Ch. Schultheiss, "Production of High Current Particle Beams by Low Pressure Spark Discharges," Zeitschrift for Physik A290, 35 (1979); D. Bloess, I. Kamber, H. Riege, G. Bittner, V. Bruckner, J. Christiansen, K. Frank, W. Hatmann, N. Lieser, Ch. Schultheiss, R. Seebock, and W. Steudtner, Nuclear Instr. & Methods 205, 173 (1983); E. Boggasch, H. Riege, and V. Bruckner, "A 400 kA Pulse Generator with Pseudospark Switches," European Organization for Nuclear Research (CERN/PS/85-30(AA)), Geneva, Switzerland, July 1985; K. Frank, E. Boggasch, J. Christiansen, A. Goertler, W. Hartmann and C. Kozlik, "High Repetition Rate Pseudospark Switches for Laser Application," Proceedings of SPIE 735, Pulse Power for Lasers, 74, 1987. See also G. Mechtersheimer and R. Kohler, "Multichannel Pseudospark Switch (MUPS)" J. Physics E: Sci. Instrum. 20, 270 (1987). A patent that is related to the prior art is U.S. Pat. No. 4,335,465, 6/982, J. Christiansen and C. Schultheiss.
As will be appreciated by those who are familiar with triggered gas discharge devices, however, the triggering methods so far described are either complex and cumbersome, or if simple, fail to produce triggering compatible with such factors as repetition rate, energy per pulse, peak current, current rate of rise, pulse to pulse jitter, delay in triggering, life, ability to conduct reverse current, and other operating conditions and requirements affected by or dependent upon triggering. Such factors, either singly or in combination, often constitute the essential limitations in the performance characteristics of the switch in a given application.
For example, attempting to trigger the device with the simple exposed wire electrode used in the first experimental pseudospark devices generally results in unacceptable time delays and jitter due to the long and erratic formative times for the long-path discharge. If triggering is wanted within a microsecond or less, as much as 7 kilovolts may be required to break down a long-path gap that under steady DC conditions will eventually self fire with only 300 or 400 volts applied. And, where a pulse-to-pulse time-jitter of less than a few nanoseconds is wanted, the initial trigger breakdown will often show jitter several hundred times as large. Increasing the tube pressure to obtain better triggering typically degrades short-path high voltage hold off between the main electrodes. Trying to enhance triggering by use of sharp points, low work function metals, high secondary-emission surfaces, dielectric discontinuities, "sparkers" and other techniques used in high pressure spark-gap triggering (i.e. in devices operating on the "right-hand side of the Paschen curve") is of little or no use.
In a similar vein, the use of, say, a mylar spark-slide or flash-board to promote low-pressure breakdown by a surface discharge, while acceptable in an unsealed laboratory prototypes attached to a pump and gas handling apparatus, is completely unacceptable in a sealed-off device, where gas purity must be maintained for thousands of hours during life.
Even the use of optical techniques, for example instantaneous optical triggering by means of ultraviolet-emitting flash-tubes, begs the triggering question somewhat, for delay and jitter associated with the flash lamp itself are then encountered, and resort must be had to delay-reducing stratagems farther back in the trigger circuit, which add further complexity to the trigger chain. One result is undue cost and complexity; another is the impracticality of "stacking" the high voltage switch devices or configuring them in other desired modes; a third is lowering of the overall "power gain" of the switch.
It is, therefore, an object of this invention to obtain triggering of a pseudospark cathode comparable to the relatively simple, generally-satisfactory triggering of hot-cathode thyratron devices without forgoing the numerous advantages of the cold cathode pseudospark or back-lighted switch. These are: essentially zero standby power, short warm up, long life, exceptionally high peak current capability without arcing, a relatively improved ability to isolate stages for the purpose of stacking, and high overall power gain.